1. Field of the Invention
The present invention relates to a control device for an internal combustion engine.
2. Related Art
In order to make an air-fuel ratio accurately equal to a target air-fuel ratio, it is necessary to accurately obtain an in-cylinder intake air amount, which is an amount of intake air sucked into a cylinder and, in particular, the in-cylinder intake air amount at a closing timing of an intake valve. There is known an internal combustion engine in which the in-cylinder intake air amount at the closing timing of the intake valve is estimated using a calculation model modeling an intake pipe which is an intake passage downstream of a throttle valve.
Use of such a calculation model will simplify the calculation. However, calculation results typically include calculation errors which should be eliminated.
Therefore, if an amount of air passing through an air flow meter is referred to as a throttle valve passing-through air amount and an air amount to be detected by the air flow meter is referred to as an air flow meter-detecting air amount, there is known an internal combustion engine in which: an air flow meter is provided for detecting an amount of air flowing through an intake passage of the engine; an in-cylinder intake air amount at the closing timing of the intake valve is estimated; a current throttle valve passing-through air amount is calculated based on a current throttle opening; a current in-cylinder intake air amount is calculated from the current throttle valve passing-through air amount and the above-mentioned calculation model; an air flow meter-detecting air amount assuming that air flows through the intake passage by the calculated current in-cylinder intake air amount is estimated; the current in-cylinder intake air amount is estimated from the estimated air flow meter-detecting air amount and the above-mentioned calculation model; the estimated in-cylinder intake air amount at the closing timing of the intake valve is corrected by a difference between the calculated current in-cylinder intake air amount and the estimated current in-cylinder intake air amount, to calculate the final in-cylinder intake air amount at the closing timing of the intake valve; and the engine is controlled using the thus calculated, final in-cylinder intake air amount at the closing timing of the intake valve (see U.S. Pat. No. 6,644,104).
The difference between the calculated current in-cylinder intake air amount and the estimated current in-cylinder intake air amount represents errors of the calculation model. Therefore, the estimated in-cylinder intake air amount at the closing timing of the intake valve corrected by the difference will represent the in-cylinder intake air amount at the closing timing of the intake valve accurately.
On the other hand, in USP'104, there is provided an air flow meter of a flow dividing type which has a bypass passage through which a part of intake air is introduced and which detects an amount of air passing through the bypass passage to thereby detect an amount of air passing through the air flow meter.
A flow area of the bypass passage is small and, therefore, the pressure loss/drop of the bypass passage should be considered when estimating the air flow meter-detecting air amount. However, in USP'104, the pressure loss of the bypass passage is not considered and, therefore, it may be impossible to accurately obtain the air flow meter-detecting air amount and thus the in-cylinder intake air amount at the closing timing of the intake valve. Accordingly, it may be impossible to control the engine accurately.